Issue with game object and script

I’m following this tutorial ((216) Unity3D Tutorial - dynamic wall creation - YouTube) about wall building and can’t get the start and end wall to show up and getting this error message, this has happed with some of the tutorials on unity learn as well.

6671875--763867--upload_2020-12-30_15-43-7.png

using System.Collections;
using System.Collections.Generic;
using UnityEngine;

public class CreateWall : MonoBehaviour
{

    bool creating;
    public GameObject start;
    public GameObject end;

    // Start is called before the first frame update
    void Start()
    {

    }

    // Update is called once per frame
    void Update()
    {
        getInput();
    }

    void getInput()
    {
        if (Input.GetMouseButtonDown(0))
        {
            setStart();
        }
        else if (Input.GetMouseButtonUp(0))
        {
            setEnd();
        }
        else
        {
            if (creating)
            {
                adjust();
            }
        }
    }

    void setStart()
    {
        creating = true;
    }
    void setEnd()
    {
        creating = false;
    }
    void adjust()
    {

    }

}

File name and class name casing doesn’t seem to match.